1. Field of the Invention
The present invention relates to a device for performing low pass, band pass, high pass as well as stop band filtering operations on electrical signals.
2. Description of the Prior Art
Filters are networks of electronic components that tailor signals to meet requirements of signal pass, phase shift, or time delay. The signal requirements imposed on filters are usually defined in terms of pass band and stop band with commonly used filters performing low pass, high pass, band pass, or stop band operations on incoming electrical signals.
An example of a second-order, low pass filter is shown in FIG. 1a and its response curve in FIG. 1e. A second-order, high pass filter and its response curve are shown in FIGS. 1b and 1f. The filter characteristics are defined by the values of C and L which are readily calculated from the following formulas: EQU C=l/w.sub.c R (1) EQU L=R/w.sub.c ( 2)
where:
C=capacitance in farads PA1 L=inductance in henries PA1 wc=cutoff frequency
R=nominal terminating resistance [=.sqroot.L/C]
A second-order, band pass filter is shown in FIG. 1c and its response curve in FIG. 1g; and a second-order, stop band filter is shown in FIG. 1d and its response curve in FIG. 1h.
Ideal filters pass frequencies within their designated pass bands with no attenuation and stop all frequencies in their stop bands with infinite attenuation. Practical filters are not ideal but, can approach the ideal if filter stages are cascaded, with the increase in stages improving the approximation in both the pass band and the stop band.
Circuit fabrication and circuit device technologies have often been determining factors in filter design procedures. Before the advent of solid state devices, filters were constructed using passive RLC (resistor, inductor, capacitor) design procedures with concomitant bulky inductors and ferromagnetic effects. With the emergence of transistors came a drastic change in design procedures from RLC filters to active RC (resistor, capacitor) filters. The availability of operational amplifiers (op-amps) on integrated circuit chips (IC) made it possible to provide all the electrical characteristics of RLC networks using only resistors and capacitors.
The attractiveness of active filters compared to their purely passive counterparts has been known and demonstrated for many years. In addition to their practical advantages of low weight, small size, and low cost, they have the theoretical advantage of easy tunability (when high-order realizations are produced by cascading second-order stages) and the ability to provide gain at the same time that a specific set of filtering characteristics is realized. The integrator gain is controlled by the dominant pole of the op-amp and by a pair of well-matched resistors in an active resistance (active R) filter or a pair of well-matched capacitors in an active capacitance (active C) filter. Such resistor or capacitors are used as voltage dividers to reduce the voltage applied to the input of the op-amp, thereby reducing the integrator gain and producing lower frequency filters.
Integration of these resistors or capacitors into an integrated circuit on the same monolithic semiconductive chip as the op-amp, however, raises several problems. It is difficult to fabricate suitable resistors using MOS processes, because of the resistivity of the diffused layers that are available to be used as resistors and/or the variability of such resistivity with applied voltage. Moreover, because the voltage divider ratios ordinarily are large, it is often difficult to achieve good matching of the layers available to be used as resistors. In contrast, capacitors can readily be formed using MOS processes, and considerably more accurate ratios can be achieved because the capacitor is ordinarily formed at the same time as the very precisely controlled gate oxide. However, it is quite difficult to provide a DC bias to the inverting input of the op-amp. This bias must come from a source impedance of greater than 100 megohms, a problem which has not been solved to date. As a result, fully integrated active C filters are not generally available.
Additional problems exist with both the active R and active C filter devices in the realization of filter roots in the frequency domain below 30 kilohertz. Large resistors or capacitors can be used to realize these roots, but they take up too much area on the chip. A MOS device could also be used in place of a large resistor, but this device introduces considerable harmonic distortion into the filter characteristics.
Another problem with both filters is that they require additional active electrical components to provide the device with stop band, zero filter characteristics.